Add coding.md into docs

Signed-off-by: Humble Chirammal <hchiramm@redhat.com>

docs/coding.md

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+# Coding Conventions
+
+Please follow coding conventions and guidelines described in the following documents:
+
+* [Go proverbs](https://go-proverbs.github.io/) - highly recommended read
+* [CodeReviewComments](https://github.com/golang/go/wiki/CodeReviewComments)
+* [Effective Go](https://golang.org/doc/effective_go.html)
+* [How to Write a Git Commit Message](https://chris.beams.io/posts/git-commit/)
+
+Here's a list of some more specific conventions that are often followed in
+the code and will be pointed out in the review process:
+
+### General
+* Keep variable names short for variables that are local to the function.
+* Do not export a function or variable name outside the package until you
+  have an external consumer for it.
+* Have setter or getter interfaces/methods to access/manipulate information in
+  a different package.
+* Do not use named return values in function definitions. Use only the type.
+  Exception: defer()'d functions.
+
+### Imports
+
+We use the following convention for specifying imports:
+
+```
+<import standard library packages>
+
+<import ceph packages>
+
+<import third-party packages>
+```
+
+Example:
+
+```go
+import (
+        "os"
+        "path"
+        "strings"
+        "time"
+
+        "github.com/ceph/ceph-csi/pkg/util"
+
+	"github.com/pborman/uuid"
+	"github.com/pkg/errors"
+
+)
+```
+
+### Error Handling
+
+* Use variable name `err` to denote error variable during a function call.
+* Reuse the previously declared `err` variable as long as it is in scope.
+  For example, do not use `errWrite` or `errRead`.
+* Do not panic() for errors that can be bubbled up back to user. Use panic()
+  only for fatal errors which shouldn't occur.
+* Do not ignore errors using `_` variable unless you know what you're doing.
+* Error strings should not start with a capital letter.
+* If error requires passing of extra information, you can define a new type
+* Error types should end with `Error`.
+
+### Logging
+
+* If a function is only invoked as part of a transaction step, always use the
+  transaction's logger to ensure propagation of request ID and transaction ID.
+* The inner-most utility functions should never log. Logging must almost always
+  be done by the caller on receiving an `error`.
+* Always use log level `DEBUG` to provide useful **diagnostic information** to
+  developers or sysadmins.
+* Use log level `INFO` to provide information to users or sysadmins. This is the
+  kind of information you'd like to log in an out-of-the-box configuration in
+  happy scenario.
+* Use log level `WARN` when something fails but there's a workaround or fallback
+  or retry for it and/or is fully recoverable.
+* Use log level `ERROR` when something occurs which is fatal to the operation,
+  but not to the service or application.